Lithium battery and electrode

ABSTRACT

An electrode includes an electrically conductive matrix containing a disulfide group, wherein an S—S bond of the disulfide group is cleaved by electrochemical reduction and reformed by electrochemical oxidation. A plurality of carbon nanotubes is dispersed in the electrically conductive matrix. The electrode can be used as a cathode of a lithium battery.

This is a continuation of Application Ser. No. 09/052,365, filed Mar.31, 1998 (abandoned) and 09/647,188, filed Sep. 27, 2000 (abandoned) andPCT/E99/01945, filed Mar. 23, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electrode, a battery precursor and alithium battery.

2. Description of Related Art

Batteries are a type of electrochemical cell containing a pair ofelectrodes and an electrolyte disposed between the electrodes. One ofthe electrodes is called a cathode wherein an active material is reducedduring discharge. The other electrode is called an anode wherein anotheractive material is oxidized during discharge. Secondary batteries referto batteries capable of charging electricity after discharge.

Recently, intensive research has been conducted on lithium secondarybatteries because of their high voltage and high energy density. Lithiumbatteries refers to batteries having an anode containing an activematerial for releasing lithium ions during discharge. The activematerial may be metallic lithium and an intercalated material beingcapable of incorporating lithium between layers.

Particular attention has been paid to an electrode material for thecathode of the lithium secondary battery. For example, U.S. Pat. No.4,833,048 discloses a cathode containing a disulfide compound forimproving an energy density. This compound is represented by R—S—S—Rwherein R is an aliphatic or an aromatic organic group and S is a sulfuratom. An S—S bond is cleaved by the electrolytic reduction in anelectrolytic cell containing cation of M⁺ to form a salt represented byR—S⁻·M⁺. This salt returns to the R—S—S—R by the electrolytic oxidation.U.S. Pat. No. 4,833,048 discloses a rechargeable battery obtained bycombining a disulfide compound with metal M which supplies and capturesthe cations (M⁺). The rechargeable battery provides an improved energydensity of at least 150 Wh/kg. The entire disclosure of U.S. Pat. No.4,833,048 is incorporated herein as reference.

However, as the inventors of U.S. Pat. No. 4,833,048 reported in J.Electrochem. Soc., Vol. 136, No. 9, pp. 2570 to 2575 (1989), thedifference between the oxidation potential and the reduction potentialof the disulfide compound is very large. For example, when[(C₂H₅)₂NCSS—]₂ is electrolyzed, the oxidation potential differs fromthe reduction potential by 1 V or more. According to the theory ofelectrochemical reaction, the electron transfer of the disulfidecompound proceeds extremely slowly at room temperature. Therefore, it israther difficult to obtain a rechargeable battery providing a highercurrent output of 1 mA/cm² or more at room temperature. The operation ofa battery comprising an electrode of disulfide compound is limited tohigh temperatures in the range of 100° to 200° C., where the electrontransfer can proceed faster.

U.S. Pat. No. 5,324,599 discloses a cathode for a lithium secondarybattery containing the disulfide compound and a conductive polymer. Theconductive polymer allows to operate the battery in much lowertemperatures such as room temperature. The entire disclosure of U.S.Pat. No. 5,324,599 is incorporated herein as reference.

Japanese Patent No. 2,513,418, which corresponds to JP-A-5-175929,discloses a cathode containing carbon nanotubes. The carbon nanotubesare obtained by electric discharge between a pair of carbon rods.Japanese Patent No. 2,513,418 does not teach the disulfide compound. Theentire disclosure of Japanese Patent No. 2513418 is incorporated hereinas reference.

WO 95/07551 discloses an electrode, which may be used for a lithiumsecondary battery, containing carbon nanotubes. The carbon nanotubes areobtained by catalytic reactions. The document further disclosesaggregates of carbon nanotubes disentangled by an ultrasonichomogenizer. The entire disclosure of WO 95/07551 is incorporated hereinas reference.

SUMMARY OF THE INVENTION

According to the first aspect of the present invention, there isprovided an electrode, comprising: an electrically conductive matrixcontaining a disulfide group, wherein an S—S bond of the disulfide groupis cleaved by electrochemical reduction and reformed by electrochemicaloxidation; and a plurality of carbon nanotubes being dispersed in theelectrically conductive matrix.

Preferably, the electrically conductive matrix may contain anelectrically conductive polymer and an organic compound having thedisulfide group. Alternatively, the electrically conductive matrix maycontain an electrically conductive polymer having the mercapto groupwhich is capable of forming disulfide group.

According to the second aspect of the present invention, there isprovided a battery precursor, comprising: a cathode having: anelectrically conductive matrix containing a disulfide group, wherein anS—S bond of the disulfide group is cleaved by electrochemical reductionand reformed by electrochemical oxidation; and a plurality of carbonnanotubes being dispersed in the electrically conductive matrix; and acathode current collector; wherein the cathode is coated onto thecathode current collector.

According to the third aspect of the present invention, a lithiumbattery, comprising: a cathode having: an electrically conductive matrixcontaining a disulfide group, wherein an S—S bond of the disulfide groupis cleaved by electrochemical reduction and reformed by electrochemicaloxidation; and a plurality of carbon nanotubes being dispersed in theelectrically conductive matrix; an anode having an active material forreleasing lithium ions; and an electrolyte being disposed between thecathode and the anode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section of a part of a homogenizer.

FIG. 2 is a photograph of a poly(methylmethacrylate) film containingdisentangled carbon nanotubes observed by transmission electronmicroscopy.

FIG. 3 is a photograph of a poly(methylmethacrylate) film containingaggregates of carbon nanotubes observed by transmission electronmicroscopy.

FIG. 4 is a cross section of a laminated structure used for a lithiumbattery.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An electrode of the present invention includes an electricallyconductive matrix containing a disulfide group. In one embodiment, theelectrically conductive matrix contains an electrically conductivepolymer and an organic compound having the disulfide group. In anotherembodiment, the electrically conductive matrix contains an electricallyconductive polymer having the mercapto group.

The disulfide group is responsible for the electrochemical reactions atthe electrode. Namely, an S—S bond of the disulfide group is cleaved byelectrochemical reduction and reformed by electrochemical oxidation.When the electrode is used as a cathode for a lithium battery, theelectrochemical reactions of the cathode and an anode are shown in theformula as follows:

wherein R—S—S—R is an organic compound having the disulfide group, R isan aliphatic or an aromatic organic group and S is a sulfur atom.

In this example, metallic lithium is used as the anode although theanode of a lithium battery is not limited to metallic lithium. When thelithium battery discharges, electrochemical reduction occurs at thecathode, and the organic compound containing the disulfide group reactswith lithium ions to cleave an S—S bond of the disulfide group thereofand to form a salt represented by R—S⁻·Li⁺. During the discharge,electrochemical oxidation occurs at the anode, and a metallic lithium isoxidized to a lithium ion.

When the lithium battery is charged, the reactions proceed in reversedirections. Specifically, electrochemical oxidation occurs at thecathode, and the salt returns to the R—S—S—R; electrochemical reductionoccurs at the anode, and the lithium ion returns to the metalliclithium.

Examples of the organic compound having the disulfide group are shown inTables 1 and 2.

Name Formula 2-mercaptoethyl ether (—SCH₂CH₂OCH₂CH₂S—)_(n) dimercaptodithiazole

dimethyl ethylenediamine

ethylenediamine

polyethylene imine derivative

trithiocyanuric acid

piperazine

2,4-dithiopyrimidine

1,2-ethanedithiiol (—SCH₂—CH₂S—)_(n) 2-mercaptoethyl sulfide(—SCH₂CH₂SCH₂CH₂S—)_(n)

Preferably, the organic compound contains a 5 to 7 membered,heterocyclic ring containing 1 to 3 heteroatoms consisting of a nitrogenatom and a sulfur atom. The heterocyclic ring may be saturated orunsaturated. Preferably, the heterocyclic ring is saturated. Furtherpreferably, the organic compound contains a thiadiazole ring andparticularly 1,3,4-thiadiazole ring. For example, a dimer of2,5-dimercapto-1,3,4-thiadiazole may be used as the organic compoundcontaining the disulfide group.

The electrically conductive polymer in use with the organic compoundcontaining the disulfide group preferably has a π electron conjugatedstructure. Examples of such electrically conductive polymer includepolymers obtained by polymerizing thiophene, pyrrole, aniline, furan,benzene, or the like. More specifically, examples of the polymersinclude polyaniline, polypyrrole, polythiophene, and polyacene. These πelectron conjugated electrically conductive polymers are reduced andoxidized with a high reversibility in 0 to ±1.0 V versus Ag/AgClelectrode. Electrically conductive polymers which are doped with anionssuch as iodine exhibit excellent properties.

The electrically conductive matrix may have a porous fibril structure.For example, electrically conductive polymers may have a porous fibrilstructure, which depends on the polymerization conditions. In otherwords, electrically conductive polymers may have a form of a pluralityof fibrils defining pores therebetween. The disulfide compound may beheld in the pores formed by the fibrils. Such electrically conductivepolymer having the porous fibril structure may be obtained bypolymerization at the electrode.

Alternatively, the electrically conductive matrix may be continuousbeing substantially free of pores. Such electrically conductive matrixmay be obtained by a standard chemical polymerization reaction.

Among the π electron conjugated electrically conductive polymers, apolymer represented by a formula:

—[Ar—NH]_(n)—

wherein Ar is aryl, and n is an integer is preferably used. Arylpreferably has carbon atoms ranging from 6 to 20, and further preferablyfrom 6 to 10. Aryl may be phenyl, naphthalenyl, indenyl and the like.Polyaniline wherein aryl is phenyl is preferred.

The matrix containing the combination of the above-mentioned disulfidecompound and an electrically conductive polymer may be produced by awell known method such as mixing, impregnating, or coating. For example,a fibril layer of the electrically conductive polymer is formed on astainless steel substrate by electrolytic polymerization, after which asalt in the disulfide compound is impregnated in the fibril layer,thereby obtaining a composite electrode. Alternatively, the disulfidecompound particles are dispersed in a solvent in which the electricallyconductive polymer is dissolved, and after that, the solvent is removed,whereby a layer of the electrically conductive polymer is formed on thesurface of the disulfide compound particle. Furthermore, theelectrically conductive polymer particles obtained by the chemicalpolymerization or electrolytic polymerization can be mixed with thedisulfide compound particles.

As another method, the electrode material of the present invention canbe obtained by polymerizing a monomer capable of forming a π electronconjugated electrically conductive polymer in the presence of thecompound containing a disulfide group therein and having a conformationwhich enables a reversible cleavage of an S—S bond of the disulfidegroup in its molecule (e.g., 1,8-disulfide naphthalene). For example,when aniline is subjected to the electrolytic polymerization on anelectrode in the presence of 1,8-disulfide naphthalene, a composite filmof polyaniline and 1,8-disulfide naphthalene is formed.

Alternatively, as another method, a dimer of a compound having amercapto group can be used instead of a compound having a conformationwhich enables a reversible cleavage of an S—S bond in its molecule. Forexample, a dimer of 2-mercapto-2-thiazoline is obtained, and apolyaniline-2-mercapto-2-thiazoline dimer composite film can be formedby using this dimer instead of 1,8-disulfide naphthalene. In any of theabove cases, it is preferred that the polymerization is conducted underthe conditions that a film having a fibril structure can be formed. Inthese methods, the compound in which a mercapto group is protected isused, so that the electrically conductive polymer can be preparedwithout any inhibition. In the composite material thus obtained, thedisulfide compound and the electrically conductive polymer forms acomposite, thereby preventing the disulfide compound from leaking out ofthe composite film into the electrolyte during its use as a cathode of arechargeable battery.

In the electrode of the present invention, a conductive polymercontaining a mercapto group may be used. The conductive polymer having amercapto group can be obtained, for example, by (1) introducing amercapto group into a π electron conjugated conductive polymer; or (2)electrolytic polymerization of a monomer having a mercapto group andbeing capable of forming a π electron conjugated conductive polymer.

As the π electron conjugated conductive polymer in this method (1), aconductive polymer or derivatives thereof used for the first electrodematerial can be used. For example, halogenated pyrrole is subjected tothe electrolytic polymerization to from a thin film of polyhalopyrroleon an electrode. At this time, it is preferred that the polymerizationis conducted in the same way as in the case of the first electrodematerial under the conditions that a thin film having a fibril structureis formed. Then, a halogen group is converted into a mercapto group bythiourea to form polypyrrole having a mercapto group. After that, acompound having a mercapto group is reacted with the polypyrrole havinga mercapto group to form polypyrrole having a disulfide group. As thecompound having a mercapto group, the disulfide compound (which is areduced form and has an SH group) used for the first electrode material,for example, 2,5-dimercapto-1,3,4-thiadiazole is preferably used. Theconductive polymer in a thin film shape having a disulfide group soobtained can be used as a reversible electrode.

As the monomer capable of forming a π electron conjugated conductivepolymer in this method (2), the monomer (e.g., thiophene and pyrrole) inwhich a disulfide group is introduced and capable of forming aconductive polymer used in the first electrode material can be used. Aconductive polymer having a having a mercapto group can be obtained bypolymerizing this monomer. For example, a thiophene derivative having adisulfide group can be obtained by reacting thiophene having a mercaptogroup with the disulfide compound which is a reduced form and has an SHgroup. The thiophene derivative having a disulfide group thus obtained(e.g., 2,5-dimercapto-1,3,4-thiaziazole) is used for the firstelectrode. This thiophene derivative is subjected to the electrolyticpolymerization on an electrode, whereby a conductive polymer film havinga disulfide group can be formed. It is preferred that the polymerizationis conducted under such conditions that a film having a fibril structureis formed. The conductive polymer film thus formed functions as areversible electrode.

In the electrode of the present invention, a plurality of carbonnanotubes are dispersed in the electrically conductive matrix. Thecarbon nanotubes conducts electricity along the axial direction thereof,thereby decreasing electric resistance of the matrix. Typically, thecarbon nanotubes has less resistance and conducts more electricity thanthe electrically conductive polymer. Moreover, the presence of thecarbon nanotubes serving as a filler increases mechanical strength ofthe matrix. Carbon nanotubes are graphitic fibers having a microscopictubular structure. While carbon nanotubes are graphitic, geometricconstraints force some differences from pure graphite. Like graphite,carbon nanotubes are composed of parallel layers of carbon but in theform of a series of concentric tubes disposed about the longitudinalaxis of the fibers rather than as multi-layers of flat graphite sheets.Thus, because of the geometric constraints in the narrow diameter of thecarbon nanotubes, the graphite layers cannot line up precisely withrespect to the layers below as flat graphite sheets can.

Ideally, a carbon nanotube consists of one or more seamless cylindricalshells of graphitic sheets. In other words, each shell is made of sp²(trivalent) carbon atoms that from a hexagonal network without anyedges. A carbon nanotube can be thought of as a tubular microcrystal ofgraphite. The tube is typically closed at each end by the introductionof pentagons in the hexagonal network. Multishell nanotubes may haveinterlayer spacing of about 0.34 nanometers and typical of turbostraticgraphite, in which the position of each layer relative to the next isnot correlated. A given nanotube will be composed of shells havingdifferent helicities. In fact, the different degrees of helicity in eachshell are necessary to obtain the best fit between the successive shellsin a tube and minimize the interlayer distance. The carbon nanotubes maybe catalytically prepared. The process provides aggregatesuncontaminated with amorphous carbon allowing carbon nanotubes to befashioned into a product with only minimal processing. The carbonnanotubes are grown by contacting catalyst particles with gaseoushydrocarbon in a hydrogen rich atmosphere. Their diameters may beaverage 7 to 12 nanometers. Lengths may be several micrometers. They arehollow tubes with wall thicknesses of 2 to 5 nanometers. These walls areessentially concentric tubes of individual graphite layers rolled intocylinders. At intervals along the length of a fiber some of the innerlayers may curve into hemispherical septa spanning the hollow interior.Near these, the walls may for a short distance change into nested cones.These reflect changes in the catalyst/carbon interface during growth ofthe fibril. Unlike other catalytic vapor grown carbon fibers they arefree of less organized pyrolytic carbon on their surfaces.

Carbon nanotubes may be prepared by condensation of carbon vapor in anarc. The carbon vapor may be produced by irradiating laser onto acarbon-nickel-cobalt mixture at 1200° C. as reported in Science Vol.273, Jul. 26, 1996 page 483. They usually have a wider distribution ofdiameters from single layer walls to many tens of layers. Some have onlyconcentric cylinders (or polygonal cross sections). Others also havesepta and nested cones. Less organized carbon is deposited at the sametime in the form of polygons or turbostratic carbon some of which maycoat the carbon nanotubes.

The carbon nanotubes prepared by condensation of carbon vapor in an arcare commercially available from Materials & Electrochemical ResearchCorporation and from its distributor of Science Laboratory Incorporationat Matsudo, Chiba, Japan. The carbon nanotubes from Materials &Electrochemical Research Corporation may have average lengths rangingfrom 0.199 μm to 2.747 μm, and average diameters ranging from 18.5 nm to38.7 nm. The carbon nanotubes contain some non-tubular carbon particlesas well. In one sample, carbon nanotubes have length of 0.843±0.185 μm,diameter of 19.6±3.7 nm, and an aspect ratio of 47.2±11.7. Such carbonnanotubes may be used in the present invention.

As would be expected from their structure and similarity to graphite,carbon nanotubes are conductive. While the conductivity of individualcarbon nanotubes is difficult to measure, a recent attempt has yieldedan estimated resistivity value of 9.5 (±4.5) m Ω cm, a resistivityslightly higher than typically measured for graphitized carbon.

The diameter of the carbon nanotubes that are used in this invention maybe 3.5 to 200 nm, and, preferably, 5 to 30 nm and their length should beat least greater than 5 times their diameter, and preferably, 10² to 10⁴times their diameter.

When the diameter of the carbon nanotubes exceeds 200 nm, their effectin providing conductivity is decreased. When it is less than 3.5 nm, thecarbon nanotubes may scatter and become difficult to handle. When thelength of the carbon nanotubes is less than 5 times their diameter,conductivity is reduced.

The aspect ratio of each of the carbon nanotubes may ordinarily begreater than 5, preferably, greater than 100, and, more preferably,greater than 1000.

The carbon nanotubes that are used in this invention can be obtained,for example, using carbon nanotubes manufactured by the method describedin Japanese Patent Application No. 2-503334 [1990] as the raw material.This material may be use in unaltered from or be subjected to chemicalor physical treatment, after which it is subjected to pulverizationtreatment. The chemical or physical treatment may be carried out beforeor after the pulverization treatment.

Examples of physical or chemical treatments of the carbon nanotubesinclude oxidation with nitric acid, oxidation with ozone, organic plasmatreatment, coating with resins such as epoxy resins and treatment withcoupling agents such as organic silicon and titanium compounds. Thephysical treatment further includes providing a sheer force onto aliquid containing aggregate of carbon nanotubes, thereby disentanglingthe aggregate.

In the present invention, carbon nanotubes in the form of aggregate maybe used. Alternatively, disentangled carbon nanotubes may be used.

When the electrolytic reduction is conducted in the presence of metalions or protons at the electrode made of the present invention, the S—Sbond of the disulfide group of the electrode material is cleaved to forma sulfur-metal ion bond or a sulfur-proton bond. The resulting electrodeis subjected to electrolytic oxidation, and the sulfur-metal ion bond orsulfur-proton bond returns to the S—S bond. The electrolytic oxidationand the electrolytic reduction involves electron transfers, which arefacilitated by carbon nanotubes in the electrically conductive matrix.

Examples of the metal ion include an alkali metal ion and an alkalineearth metal ion. In the case where the electrode made of the electrodematerial of the present invention is used as a cathode, and a lithiumion is used as the alkali metal ion; when an electrode made of lithiumor a lithium alloy such as lithium-aluminum is used as an anode whichsupplies and captures lithium ions, and an electrolyte which cantransmit lithium ions is used, a battery having a voltage of 3 to 4 Vcan be obtained. When an electrode made of a hydrogen storage alloy suchas LaNi₅ is used as an anode which supplies and captures protons, and anelectrolyte which can conduct protons is used, a battery having avoltage of 1 to 2 V can be obtained. In the combination of the disulfidecompound and the π electron conjugated electrically conductive polymer,the π electron conjugated electrically conductive polymer functions asan electrode catalyst for the electrolytic oxidation and reduction ofthe disulfide compound. In the case of the π electron conjugatedelectrically conductive polymer having a disulfide group, when thedisulfide group is subjected to the electrolytic oxidation andreduction, the electronic structure given by the conjugated π electronfunctions as an electrode catalyst. In the case of the disulfidecompound alone, the difference between the oxidation potential and thereduction potential is 1 V or more. However, in the case of using acombination of the π electron conjugated electrically conductive polymerand the disulfide compound, or the electrically conductive polymerhaving a disulfide group, the difference between the oxidation potentialand the reduction potential is reduced to 0.1 V or less. In thedisulfide compound which is combined with the π electron conjugatedelectrically conductive polymer or which is introduced into such apolymer, the electrode reaction is promoted and a higher current densityat room temperature is obtained on electrolysis, i.e., on charging ordischarging. When the electrode material of the present invention issubjected to electrolytic oxidation, the π electron conjugatedelectrically conductive polymer (a conjugated polymer portion in thecase of the electrically conductive polymer having a disulfide group) isoxidized at first and the resulting oxidized form of the polymeroxidizes the reduced type of the disulfide compound (an SH or S-metalion portion in the case of the electrically conductive polymer having adisulfide group). Thus, the oxidized form of the π electron conjugatedpolymer returns to the reduced form and an oxidized form of thedisulfide compound is generated (i.e., a disulfide group is formed).When the electrolytic reduction is first conducted, the electricallyconductive polymer is reduced and the resulting reduced form reduces theoxidized form of the disulfide compound. Thus, the reduced form of the πelectron conjugated polymer returns to the oxidized form and thedisulfide compound becomes a reduced form. The introduction of theelectrode catalyst into the disulfide compound electrode is disclosed inU.S. Pat. No. 4,833,048 or J. Electrochem. Soc., Vol. 136, pp. 2570-2575(1989). However, as the electrode catalyst, only the organic metalliccompound is disclosed. The effects of the electrode catalyst are notdescribed in detail. As described above, the π electron conjugatedpolymer or the conjugated polymer portion has a function for promotingthe movement of the electrons in the oxidation-reduction reaction. Itfunctions as a catalyst in the oxidation-reduction of disulfide,reducing the activation energy of the reaction. In addition to that, theπ electron conjugated polymer or the conjugated polymer portionincreases an effective reaction area between the electrolyte and theelectrodes.

The lithium battery of the present invention includes the cathode whichthe aforementioned electrode serves as.

An anode of the lithium battery of the present invention is not limited.The anode may contain a carbon material, and the carbon materialincludes natural graphite, artificial graphite, amorphous carbon,fibrous carbon, powdery carbon, petroleum pitch carbon, and coal cokecarbon. It is preferred that these carbon materials are particles orfibers having a diameter of 0.01 to 10 micrometers and a length of fromseveral micrometers to several millimeters.

An anode of a lithium battery may contain aluminum or an alloycontaining aluminum. Example of the aluminum or alloys thereof includesAl, Al—Fe, Al—Si, Al—Zn, Al—Li, and Al—Zn—Si. It is preferred that thealuminum or alloys thereof are flaky powders obtained by rapid cooling,or spherical or amorphous powders obtained by mechanical crushing in theair or an inactive gas such as nitrogen. The particle size is preferably1 μm to 100 μm.

The mixing ratio of the carbon material to the aluminum or aluminumalloy may be 0.01 to 5 parts by weight, preferably 0.05 to 0.5 parts byweight based on one part by weight of the aluminum or aluminum alloy.

Alternatively, the anode may be so-called rocking chair cell.Intercalated compounds such as graphite may intercalate the lithiumtherebetween.

The electrolyte of the lithium secondary battery of the presentinvention is not limited as long as the electrolyte conducts lithiumions. The electrolyte may be a liquid electrolyte, a solid electrolyteand a gel electrolyte. Preferably, the electrolyte is the solid or gelelectrolyte, and further preferably the electrolyte maintain a solidstate or a gel state at temperatures ranging from −20° to 60° C.Alternatively, a porous separator defining pores and being made of apolymer material may be disposed between the cathode and the anode, andthe liquid electrolyte may be present in the pores thereof. The liquidelectrolyte man contain a lithium salt dissolved therein. The solidelectrolyte may contain a lithium salt and preferably contain a polymercontaining the lithium salt. Examples of the salt containing lithiuminclude Lil, Li₃N—Lil—B₂O₃, Lil·H₂O, and Li—β—Al₂O₃.

For example, the solid electrolyte may be a composite of polyethyleneoxide and a lithium salt dissolved therein. In addition, the solidelectrolyte may be a poly(acrylonitrile) film comprising propylenecarbonate and LiClO₄ dissolved in the propylene carbonate.

The anode and the cathode may contain the component for the electrolyte.For example, a composition for the solid electrolyte may comprise: apolyether obtained by adding ethylene oxide and butylene oxide to apolyamine; an ion-exchangeable compound having a layered crystalstructure; and a lithium salt, and such composition may be mixed addedto a composition for the anode or the cathode.

The polyether can be obtained by the addition reaction of ethylene oxideand butylene oxide with polyamine using an alkali catalyst at 100° C. to180° C. under an atmospheric pressure of 1 to 10 atm. As the polyaminewhich is a component of the above polyether, polyethylenimine,polyalkylenepolyamine or derivatives thereof can be used. Examples ofthe polyalkylenepoly- amine include diethylenctriamine,triethylenetetramine, hexamethylenetctramine, and dipropylenetriamine.The additional number of the total moles of ethylene oxide and butyleneoxide is 2 to 150 moles per one active hydrogen of the polyamine. Themolar ratio of ethylene oxide (EO) to butylene oxide (BO) is 90/20 to10/90 (=EO/BO). The average molecular weight of the poly ether thusobtained is in the range of 1,000 to 5,000,000. It is preferred that thepolyether is contained in the solid electrode composition in an amountof 0.5 to 20% by weight. The polyether of the solid electrolyte servesas a surfactant so that this composition is uniformly dispersed.

Examples of the ion-exchangeable compound having a layered crystalstructure include clay minerals including silicate such asmontmorillonite, hectorite, saponite, and smectite, phosphoric esterssuch as zirconium phosphate and titanium phosphate, vanadic acid,antimonic acid, tungstic acid; or substances obtained by modifying theseacids with organic cations such as quaternary ammonium salts or withorganic polar compounds such as ethylene oxide and butylene oxide.

FIG. 4 is a cross section of a laminated structure used for a lithiumbattery. The structure 30 has a cathode 34, an anode 38 having an activematerial for releasing lithium ions; and an electrolyte 36 beingdisposed between the cathode 34 and the anode 38. The structure has acathode current collector 32 contacting with the cathode 34; and ananode current collector 40 contacting with the anode 38. In the presentinvention, the cathode 34 has an electrically conductive matrixcontaining a disulfide group, wherein an S—S bond of the disulfide groupis cleaved by electrochemical reduction and reformed by electrochemicaloxidation; and a plurality of carbon nanotubes being dispersed in theelectrically conductive matrix. The cathode current collector 32, thecathode 34, the electrolyte 36, the anode 38, and the anode currentcollector 40 have a layered structure and are laminated each other inthis order. The electrolyte 36 may have at least one of a solidelectrolyte and a gel electrolyte.

When the lithium secondary battery of the present invention is charged,Li is liberated from an S—Li bond of the cathode and an S—S bond isformed. On the surface of the anode or inside the anode (when the anodecomponent and the electrolyte component are mixed), lithium is uniformlydeposited. Since lithium is directly deposited from the electrolyte,impurities such as oxygen are not likely to contaminate. Accordingly,even when the charging and discharging are repeated, current is notlikely to concentrate, whereby any short-circuit in the battery can beeffectively prevented. Lithium generated during charging (electrolysis)and electrolyte are in a good contact with each other, so that thepolarization during discharging can be decreased, and a higher currentcan be realized. As described above, when the electrolyte is mixed inthe cathode and/or the anode, especially effective results can beobtained. In this case, it is particularly effective that compoundshaving lithium salts, polyether, and a layered crystal structure areused as the electrolyte.

The lithium secondary battery of the present invention can be preparedby the following method also. Firstly, aggregates of carbon nanotubesare obtained by a conventional method.

Disentangled carbon nanotubes may be obtained by a process including thesteps of: adding a plurality of aggregates of carbon nanotubes to aliquid; and providing sheer force onto the liquid for disentangling theaggregates of carbon nanotubes therein.

The liquid may have a viscosity at 25° C. of not less than 0.8centipoise and preferably not less than 1.0 centipoise since the viscoseliquid facilitates to apply sheer force by mechanical process. Theviscosity of some liquids are summarized in Table 3.

TABLE 3 liquid viscosity at 25° C. in centipoise N-methyl-2-pyrrolidone1.67  2-propanol 1.77  methanol 0.545

The liquid may be an organic solvent or water. The organic solvent ispreferably polar. Examples of the organic solvent includesN-methyl-2-pyrrolidone. When water is used, preferably, water contains asurfactant. The sheer force may be provided by a mechanical process, andthe liquid containing the aggregates may passed through a narrow gap ata high speed.

For example, a homogenizer may be used to apply the sheer force. In FIG.1, the homogenizer 10 has a stator 12 which has a radially inner surface13; and a rotor 22 which has a radially outer surface 23. The stator 12and the rotor 22 shares an axis. The radially inner surface 13 of thestator 12 and the radially outer surface 23 of the rotor 22 define anarrow gap having an arc or circular configuration therebetween. A blade26 is fixed to the rotor 22 and being disposed in the narrow gap. As therotor 22 rotates, the blade 26 rotates along the narrow gap.

The stator 12 is formed of at least one hole 14 in radial directions,allowing a liquid passing therethrough. Similarly, the rotor 22 isformed of at least one hole 24 in radial directions, allowing a liquidpassing therethrough. Typically, the liquid passes through the hole 24in radially outward directions and subsequently through the hole 14 inradially outward directions.

When the liquid has a plurality of aggregates 16, the aggregates 16 areforced to pass through the narrow gap by the blade 26 so that sheerforce is applied thereonto. The aggregate 16 is disentangled graduallyand becomes smaller particles 18.

Alternatively, the ultrasonic generator may apply ultrasonic waves ontothe liquid containing the aggregates, thereby disentangling theaggregates therein.

Preferably, a mixture containing disentangled carbon nanotubes and aliquid medium is mixed with the organic compound containing thedisulfide group and the electrically conductive polymer. Alternatively,the mixture containing disentangled carbon nanotubes and the liquidmedium may be mixed with the electrically conductive polymer containingthe mercapto group. The liquid medium may be the same as or differentfrom the liquid used for disentangling aggregates of carbon nanotubes.

A battery precursor having the current collector and the cathode filmlaminated thereon can be prepared by coating the composition for thecathode film onto the current collector, which may be metallic foil.

The structure 30 of FIG. 4 may be made from the battery precursor. Theelectrolyte 36, the anode 38 and the anode current collector 40 may belaminated onto the battery precursor.

A plurality of the structures 30 may be laminated each other and packedin a housing to produce a lithium battery. Alternatively, a plurality ofthe structures 30 may be rolled to a generally cylindricallyconfiguration, and then packed in a housing.

The lithium secondary battery of the present invention can be preparedby the following method also. Respective compositions of the cathode,the anode, and the electrolyte are molded into films. The composition ofthe cathode contains the carbon nanotubes. The cathode film, electrolytefilm, and anode film are laminated in this order and pressed together,thereby obtaining a unit cell. If required, electrically conductivefoils, serving as current collectors, and leads are attached to thecathode and the anode of this unit cell, and the assembly is packaged,thereby producing a lithium secondary battery. Preferably, theelectrolyte component is admixed in the cathode and/or the anode.

EXAMPLE Example 1

Carbon Nanotubes

Firstly, aggregates of carbon nanotubes were disentangled. Theaggregates of carbon nanotubes were added to 1-methyl-2-pyrrolidone togive a mixture containing 1 percent by weight of the carbon nanotubes.The mixture was added to a homogenizer under a trade name of ULTRA TALUXT-25 from IKA Japan Company Limited in Nakayama-ku, Yokohama, Japan. Thehomogenizer applied sheer force to the mixture, thereby disentanglingthe aggregate. The homogenizer has a structure of FIG. 1. In thehomogenizer, the rotor may rotate 8,000 to 24,000 round per minute.

Secondly, we confirmed that aggregates of carbon nanotubes weredisentangled by following procedures, which are not necessary inproducing an electrode including the disentangled carbon nanotubes. Tothe liquid mixture thus obtained, which contains one part by weight ofcarbon nanotubes, was added 19 parts by weight of polymethylmethacrylateserving as a binder and further N-methyl-2-pyrrolidone for dilution. Thepolymethylmethacrylate, which is referred to PMMA hereinafter, has aweight average molecular weight of 996,000 and is commercially availablefrom Aldrich. The liquid mixture was casted onto a glass substrate, andthe glass substrate was placed in a vacuum oven for evaporating thesolvent, thereby producing a PMMA film containing 5 percent by weight ofcarbon nanotubes. We observed the PMMA film by transmission electronmicroscopy. FIG. 2 is a photograph of the result. Fibrils, whichcorrespond to carbon nanotubes, are disentangled and dispersed in thePMMA matrix.

As a comparative example, we did not apply sheer force to the liquidmixture containing aggregates of the carbon nanotubes. Specifically, theliquid mixture containing N-methyl-2-pyrrolidone and 1 percent by weightof carbon nanotubes was mixed by a magnetic stirrer overnight. AnotherPMMA film was produced in the same manner as mentioned above using theresultant liquid mixture, and the PMMA film was observed by transmissionelectron microscopy. FIG. 3 is a photograph of the result. A pluralityof aggregates of the carbon nanotubes are present in the matrix.

Battery Precursor

1.8 gram of a powder of 2,5-dimercapto-1,3,4-thiadiazole was mixed with1.2 gram of polyaniline by a ball mill. To 2.5 gram of the powdermixture was added 11.1 gram of a liquid mixture containing 2 percent byweight of disentangled carbon nanotubes in N-methyl-2-pyrrolidone, andthe resultant mixture was mixed in a mortar to produce an ink. The inkwas coated onto a copper foil having a thickness of 35 micrometers usinga doctor blade having a gap of 200 micrometers. The copper foil wasplaced in a vacuum oven at 80° C. for 3 hours for drying the inktherein, thereby producing a battery precursor having the copper foiland the film serving as a cathode and having a thickness of about 40micrometers coated thereon.

The resistivity of the film was determined by an instrument fordetermining resistivity, which has a trade name of K-705RS and which iscommercially available from Kyowa Riken. The resistivity of the film was40 ohm per square centimeter.

The adhesion of the film onto the copper foil was determined by a gridtape test in accordance with Japan Industrial Standard K 5400 8.5.2. Thetest provided 6 to 8 points, which indicates that the film did notadhere to the tape and that the film adhered to the copper foil.

Hardness of the film having a thickness of 20 micrometers onto thecopper foil was determined by scrabbling a surface of the film by apencil having a hardness of 8H in accordance with Japan IndustrialStandard K 5400 8.4.1. The scrabbling hardly damage the surface of thefilm. The film was folded along with the copper foil. However, the filmneither peel off nor crack. The result shows that the film maintainsflexibility, which is critical to manufacturing a lithium battery.

Comparative Example 1

As a comparative example, a battery precursor having the copper foil andthe film serving as a cathode coated thereon was produced in the samemanner except that carbon nanotubes were replaced by ketjen black, whichis commercially available from Akzo.

The resistivity of the film was determined by the same instrument to be50 kiloohm per square centimeter.

The adhesion of the film was determined by the same grid tape test inaccordance with Japan Industrial Standard K 5400 8.5.2, and the testprovided 0 point, which indicates that the film peeled off along withthe tape from the copper foil.

The scrabbling test of the film having a thickness of 55 micrometersonto the copper foil in accordance with Japan Industrial Standard K 54008.4.1 showed that a soft pencil having a hardness of HB damages thesurface of the film. The result shows that the film incorporating ketjenblack is much softer than the film incorporating carbon nanotubes.

Example 2

Lithium Secondary Battery

A lithium secondary battery having a coin configuration was produced.The aforementioned battery precursor was cut to a disk configurationhaving a diameter of 16 mm and used as the cathode.

A gel electrolyte was obtained by a method as follows. To a mixture of14.5 gram of propylene carbonate and 25.1 gram of ethylene carbonate wasadded 4.8 gram of lithium tetrafluoroborate. A powder of 5.0 gram of acopolymer of polyacrylonitrile and polymethylacrylate, which wascommercially available from Scientific Polymer Product and has a weightaverage molecular weight of 100,000. The mixture thus obtained wasstirred by magnetic stirrer for one day to obtain a polymeric dispersionhaving a white color. The polymeric dispersion was placed in a tray madeof stainless steel, and heated to 125 degree Celsius to obtain acolorless dispersion. Meanwhile, onto a glass sheet was placed a pair ofTEFLON sheets with a thickness of 0.5 mm at both ends of the glasssubstrate. The polymeric dispersion, which is colorless and flowable,was added to the glass substrate between the TEFLON sheets. Anotherglass sheet is placed onto the glass sheet, and the pair of glass sheetswere cooled to room temperature. Subsequently, the glass sheets wasfurther cooled in a freezer, and then warmed back to room temperature.The gel film thus obtained was cut in a circular configuration having adiameter of 18 mm.

A foil made of metallic lithium was used as the anode, and the copperfoil was used as the anode current collector.

The battery precursor, the gel electrolyte, the anode, and anode currentcollector were laminated in this order.

The lithium secondary battery having a coin configuration was subject torepeated cycles of discharging and charging. It turned out that thelithium battery maintained more than 90 percent of discharging capacityafter 100 cycles of discharging and charging.

An electrode of the present invention has improved electric conductivityand mechanical strength. Compared to other carbon materials, a smalleramount of carbon nanotubes allow to maintain necessary electricalconductance and mechanical strength of the electrode.

A battery precursor of the present invention has improved adhesion tothe current collector.

The electrode of the present invention is suitable for a cathode of alithium battery and particularly a lithium secondary battery. Theelectrode may be used in a sensor for detecting an electric potential ofa medium.

What is claimed is:
 1. An electrode, comprising: an electricallyconductive matrix containing a disulfide group, wherein an S—S bond ofthe disulfide group is cleaved by electrochemical reduction and reformedby electrochemical oxidation; and a plurality of carbon nanotubes beingdispersed in the electrically conductive matrix.
 2. An electrode ofclaim 1 wherein the electrically conductive matrix contains anelectrically conductive polymer and an organic compound having thedisulfide group.
 3. An electrode of claim 2 wherein the electricallyconductive polymer comprises a polymer represented by a formula:—[Ar—NH]_(n)— wherein Ar is aryl, and n is an integer.
 4. An electrodeof claim 2 wherein the electrically conductive polymer comprisespolyaniline.
 5. An electrode of claim 2 wherein the organic compoundcontains a 5 to 7 membered, heterocyclic ring containing 1 to 3heteroatoms consisting of a nitrogen atom and a sulfur atom.
 6. Anelectrode of claim 2 wherein the organic compound contains a thiadiazolering.
 7. An electrode of claim 1 wherein the electrically conductivematrix contains an electrically conductive polymer having the mercaptogroup which is capable of forming disulfide group.
 8. An electrode ofclaim 1 wherein the electrode contains 0.5 to 6 percent by weight of thecarbon nanotubes based on a sum of the electrically conductive matrixand the carbon nanotubes.
 9. An electrode of claim 1 wherein theelectrode contains 1 to 4 percent by weight of the carbon nanotubesbased on a sum of the electrically conductive matrix and the carbonnanotubes.
 10. An electrode of claim 1 wherein the carbon nanotubes havean average diameter of 3.5 to 200 nanometers and an average length of0.1 to 500 micrometers.
 11. An electrode of claim 1 wherein the carbonnanotubes have an average diameter of 5 to 30 nanometers and an averagelength of 100 to 10000 times the diameter thereof.
 12. A batteryprecursor, comprising: (a) a cathode having: an electrically conductivematrix containing a disulfide group, wherein an S—S bond of thedisulfide group is cleaved by electrochemical reduction and reformed byelectrochemical oxidation; and a plurality of carbon nanotubes beingdispersed in the electrically conductive matrix; and (b) a cathodecurrent collector; wherein the cathode is coated onto the cathodecurrent collector.
 13. A battery precursor of claim 12 wherein thecathode current collector and the cathode have a layered structure. 14.A battery precursor of claim 12 wherein the cathode has a thicknessranging from 5 to 500 micrometers.
 15. A battery precursor of claim 12wherein the cathode has a thickness ranging from 10 to 100 micrometers.16. A battery precursor of claim 12 wherein the cathode currentcollector has a sheet configuration.
 17. A battery precursor of claim 12wherein the cathode current collector comprises a metallic foil.
 18. Abattery precursor of claim 12 wherein the electrically conductive matrixcontains an electrically conductive polymer and an organic compoundhaving the disulfide group.
 19. A battery precursor of claim 18 whereinthe electrically conductive polymer comprises a polymer represented by aformula: —[Ar—NH]_(n)— wherein Ar is aryl, and n is an integer.
 20. Abattery precursor of claim 18 wherein the organic compound contains a 5to 7 membered, heterocyclic ring containing 1 to 3 heteroatomsconsisting of a nitrogen atom and a sulfur atom.
 21. A battery precursorof claim 12 wherein the electrically conductive matrix contains anelectrically conductive polymer having the mercapto group which iscapable of forming the disulfide group.
 22. A battery precursor of claim12 wherein the cathode contains 0.5 to 6 percent by weight of the carbonnanotubes based on a sum of the electrically conductive matrix and thecarbon nanotubes.
 23. A battery precursor of claim 12 wherein the carbonnanotubes have an average diameter of 3.5 to 200 nanometers and anaverage length of 0.1 to 500 micrometers.
 24. A lithium battery,comprising: (a) a cathode having: an electrically conductive matrixcontaining a disulfide group, wherein an S—S bond of the disulfide groupis cleaved by electrochemical reduction and reformed by electrochemicaloxidation; and a plurality of carbon nanotubes being dispersed in theelectrically conductive matrix; (b) an anode having an active materialfor releasing lithium ions; and (c) an electrolyte being disposedbetween the cathode and the anode.
 25. A lithium battery of claim 24,further comprising: (d) a cathode current collector contacting with thecathode; and (e) an anode current collector contacting with the anode.26. A lithium battery of claim 25 wherein the cathode current collector,the cathode, the electrolyte, the anode, and the anode current collectorhave a layered structure and are laminated each other in this order. 27.A lithium battery of claim 24, wherein the electrolyte comprises atleast one of a solid electrolyte and a gel electrolyte.
 28. A lithiumbattery of claim 24 wherein the electrically conductive matrix containsan electrically conductive polymer and an organic compound having thedisulfide group.
 29. A lithium battery of claim 28 wherein theelectrically conductive polymer comprises a polymer represented by aformula: —[Ar—NH]_(n)— wherein Ar is aryl, and n is an integer.
 30. Alithium battery of claim 28 wherein the organic compound contains a 5 to7 membered, heterocyclic ring containing 1 to 3 heteroatoms consistingof a nitrogen atom and a sulfur atom.
 31. A lithium battery of claim 24wherein the electrically conductive matrix contains an electricallyconductive polymer having the mercapto group which is capable of formingdisulfide group.
 32. A lithium battery of claim 24 wherein the cathodecontains 0.5 to 6 percent by weight of the carbon nanotubes based on asum of the electrically conductive matrix and the carbon nanotubes. 33.A lithium battery of claim 24 wherein the carbon nanotubes have anaverage diameter of 3.5 to 200 nanometers and an average length of 0.1to 500 micrometers.
 34. A lithium battery of claim 24 wherein thecathode has a thickness ranging from 5 to 500 micrometers.